[java-storage] Storage.create methods don't set `content-length` header even if the size is known

I was looking at the underlying HTTP requests made by this program:
```java
import static java.nio.charset.StandardCharsets.UTF_8;

import com.google.cloud.storage.BlobId;
import com.google.cloud.storage.BlobInfo;
import com.google.cloud.storage.StorageOptions;
import java.util.logging.Level;
import java.util.logging.Logger;

class X {
private static final String BUCKET_NAME = "mybucket";

public static void main(String[] args) throws Exception {
Logger.getLogger("com.google.api.client.http").setLevel(Level.ALL);
Logger.getLogger("").getHandlers()[0].setLevel(Level.ALL);
try (var s = StorageOptions.getDefaultInstance().getService()) {
var b = BlobId.of(BUCKET_NAME, "blob");
s.create(
BlobInfo.newBuilder(b).setContentType("text/plain").build(),
"hello world".getBytes(UTF_8));
s.delete(b);
}
}
}
```
The actual object insertion request comes out like this:
```
POST https://storage.googleapis.com/upload/storage/v1/b/mybucket/o?projection=full&uploadType=multipart
Accept-Encoding: gzip
Authorization: Bearer redacted
User-Agent: gcloud-java/2.61.0 Google-API-Java-Client/2.7.2 Google-HTTP-Java-Client/2.0.2 (gzip)
x-goog-user-project:
x-goog-api-client: gl-java/21.0.9 gdcl/2.7.2 linux/6.17.0 gccl-invocation-id/2596662c-8ab2-4ff3-8bae-d7859bee9f6c
x-goog-gcs-idempotency-token: 2596662c-8ab2-4ff3-8bae-d7859bee9f6c
Content-Type: multipart/related; boundary=__END_OF_PART__3a425b6d-f309-4ccf-99cd-3a84be601630__
Content-Encoding: gzip

boundary=__END_OF_PART__3a425b6d-f309-4ccf-99cd-3a84be601630__' -H 'Content-Encoding: gzip' -d '@-' -- 'https://storage.googleapis.com/upload/storage/v1/b/mybucket/o?projection=full&uploadType=multipart' << $$$
Jan 12, 2026 4:12:05 PM com.google.api.client.util.LoggingByteArrayOutputStream close
CONFIG: Total: 449 bytes
Jan 12, 2026 4:12:05 PM com.google.api.client.util.LoggingByteArrayOutputStream close
CONFIG: --__END_OF_PART__3a425b6d-f309-4ccf-99cd-3a84be601630__
Content-Length: 94
Content-Type: application/json; charset=UTF-8
content-transfer-encoding: binary

{"bucket":"mybucket","contentType":"text/plain","crc32c":"yZRlqg==","name":"blob"}
--__END_OF_PART__3a425b6d-f309-4ccf-99cd-3a84be601630__
Content-Type: text/plain
content-transfer-encoding: binary

hello world
--__END_OF_PART__3a425b6d-f309-4ccf-99cd-3a84be601630__--
```

I noticed that the object body part doesn't include a `Content-Length` header even though that is known to the storage SDK.

Start at the Java Storage.create methods shown in the issue and trace how the multipart upload request is assembled. Reproduce the request with a known byte-array size, then verify that the object body part includes a Content-Length header and add coverage for the affected create path.

索引モデルが issue の本文から書いたものです。